Benchmark angle-differential cross-section ratios for the electron-impact excitation of the xenon 5p6 1S0→5p56s[3/2]2 , 5p56s[3/2]1 , 5p56s′[1/2]0 , and 5p56s′[1/2]1+5p56p[1/2]1 transitions at low and near-threshold incident electron energies